Swim Devices

ABSTRACT

A swim device comprising elongated foot pockets which surround each of the feet of the swimmer, continued distally with tapers ending in neckings connected in a functional manner to blades for each foot. The neckings of the tapers of both feet may be connected to one blade (monofin). A swim device comprising a cowling of generally elongated shape which surrounds both feet of the swimmer, continued distally with a taper, ending in a necking to which one blade (monofin) is connected in a functional manner. The tapers and the hydrodynamic cross-sections of the foot pockets and of the cowling, result in stronger thrust and lower drag than in the designs of prior art.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention refers to devices which increase the swimming speed when attached to the feet or legs, flapping in tandem (fins) or in unison (monofins), of swimmers.

2. Prior Art

Swim devices, whether fins or monofins, are fastened to the feet of the swimmer, who imposes on them an oscillating motion (flapping), usually in the sagittal plane. In general terms, the swim device consists of a “blade” which is fastened to the feet of the swimmer and may be provided with “pockets”, “shoes” or similar devices which accommodate the feet of the swimmer, to which it is attached. The blade and the feet are fastened to each-other in a variety of ways, sometimes in a fixed relationship, other times in a manner which allows a relative movement or re-orientation. The strength of the thrust produced is function of the speed and amplitude of the oscillatory movement of the legs and size, shape and orientation of the swim device.

The shapes of most of the blades vary between two extremes. On one side, the shape of the blade is modeled after the flukes of cetaceans, mainly dolphins, in terms of shape, size, cross section, elasticity. A few typical devices, out of the numerous patents belonging to this class are: U.S. Pat. No. 5,429,536 (Evans), U.S. Pat. No. 6,086,440 (Fechtner), U.S. Pat. No. 6,183,327 (Meyer), U.S. Pat. No. 7,510,453 (Nguyen). One of the advanced designs of this group of swim devices is the Lunocet [Adventure, September 2008, p. 21]. The design was based on detailed measurements of the shape, profile, texture, of the cetacean flukes as well as of advanced methods of mathematical modeling. The result is a swim monofin having a span of approx. 1 m, provided with the means to be fastened to the feet of the swimmer. The legs of the swimmer are close and parallel to each-other. Swimmers using the Lunocet claim that they can achieve a speed by 50% higher than when using regular (one on each foot) fins. On the other extreme, the blade is shaped as a hydrofoil, with a more or less straight (un-swept) leading edge, incorporating the features which will confer to it high lift and low drag. The blade has hydrodynamic cross section (hydrofoil) and high aspect ratio, such as in U.S. Pat. No. 4,781,637 (Caires), U.S. Pat. No. 6,881,113 (2005) to Smith, U.S. Pat. No. 7,988,508 (2011) to Langenfeld et al. Such monofins are attached to the feet of the swimmer, by means of a pair of shoes, foot pockets, provided with straps, buckles or similar devices. Hundreds of patents for fins and monofins have been issued over the years, by the US Patent Office. Only a few typical patents which are somewhat relevant to the present invention, will be reviewed.

U.S. Pat. No. 4,689,029 (1987), U.S. Pat. No. 4,767,368 (1988), U.S. Pat. No. 4,869,696 (1989), all to Ciccotelli, refer to swim fins which basically consist of a foot pocket to which a rigid blade is pivotally attached by means of beams, such that a free space is provided between the foot pocket and the blade.

The exterior of the foot pocket has basically a rectangular plane form. The slim beams connecting the foot pocket to the blade will reduce somewhat the resistance to movement through the water. Still, the shape of the foot pocket will produce a high drag.

U.S. Pat. No. 3,665,535 (1972) and U.S. Pat. No. 4,934,971 (1990) both to Picken, refer to swim fins in which a web, acting as the blade is attached to the shoe assembly by means of a rigid supporting frame, provided with means for limiting the amplitude of the pivotal movement of the web.

The design addresses only the shape pf the blade. No attention is given to the drag and power loss caused by the shape of the shoe assembly.

In U.S. Pat. No. 5,421,758 (1995) to Watson and Sigler, a scuba fin is described which consists of a foot pocket and a fin section, connected by a shaft of minimal thickness, so that it encounters a minimal resistance when moving through water. The cross-section of the shaft is rhombic in shape, with straight or rounded sides. However, no attempt is made to reduce the drag encountered by the foot pocket when moving through water. Also, the blade of the fin is tilted relative to the bottom surface of the foot plate, by an angle of less than 45 degrees, which is claimed to be beneficial for walking with the fins on.

The imposing of such an angle is not conducive to good swimming power efficiency. Although the shaft is intended to reduce drag, no other component of the swim fin is suitable to this objective. The drag generated due to the not hydrodynamic shapes of the foot pocket and of the swimmer's foot will result in wasting a significant fraction of the energy spent for flapping.

U.S. Pat. No. 5,460,557 (1995) to Arnold, has a fin blade positioned so that it “always has a positive angle of attack” which is connected by means of two struts having a flattened profile, to the shoe on the foot of the swimmer and to a cuff placed above and including the ankle. The blade may oscillate so that it is always positioned in the “undertow” of the flapping foot. The shoe has a “form of low flow resistance” in order to improve the “flow conditions” around the swimmer's foot. This is important “because the profile is in the undertow of the foot”. A second profile, may be placed downstream of the first profile, to which it is “elastically” connected.

The approach taken in this patent for reducing the drag encountered during flapping is to allow the blade to swing between two extreme positions, at the beginning of the up- or down-movements of the feet. The presence of the lateral struts will disturb the flow up-stream of the blade and lower the power efficiency of the device. Significant drag will result due to the position, of the feet which interferes with the water flow produced by the fin.

In U.S. Pat. No. 5,906,525 (1999) to Melius et al., describes a swim fin of modular constructions, inspired after aquatic mammals or fishes, consisting of a “heel”, a “boot” and a “wing-like” blade, mimicking the head, body and tail respectively, of a fish. The boot and the blade are connected to each-other by a “flexible caudal shaft” and extend in the plane of the swimmer's foot sole beyond the toes. The device is claimed to produce propulsion by using only “power strokes” which generate “lift” and produce a deformation of the blade and shaft, while releasing vortices from the tips of the fin. The “recoil” to the original shape is claimed to provide additional thrust.

The swim fin mimics the contour of a flattened the body of a fish, the axis of which forms a large angle with the sheens of the swimmer. This will reduce the efficiency of thrust generation.

In U.S. Pat. No. 6,183,327 (2001) to Meyer, the swim fin is made of an “attachment portion” “removably secured” to a swimmer's foot and consists of a two symmetrical halves, which basically define the blade, shaped like a dolphin fluke, which has its leading edge right at the tip of the swimmer's toes. In this design no attempt is made to reduce the drag produced by the foot attachment portion of the fin. In U.S. Pat. No. 6,375,531 (2002) to Melius, a “flat blade” attached with its narrow end to the foot of the swimmer is continued, with a “dolphin tail” connected at the center of the wider end of the flat blade. When a “proper angle of attack” is achieved, the water displaced by the flat blade is claimed to be interacting with the dolphin-tail, to produce lift and additional thrust. The subject device is claimed to efficiently generate thrust by moving water, by means of a tail shaped after the flukes of dolphins. Only a small amount of the water moved by the flat blade will follow the shown paths needed for generating thrust. Most of the moved water will not produce thrust, but will generate eddies and drag. According to the same patent, blades mimicking the shape of dolphin tails may be attached to the hands of the swimmer and used for increasing the propulsion through water.

U.S. Pat. No. 6,893,307 (2005) to Melius, covers an “ergonomic swim fin apparatus” consisting of a foot-pocket, provided laterally with two “channeling scoops” and a “flexible blade” which runs beyond the toes, parallel with the swimmer's sole, to a trailing edge′ to which a wing shaped tail fin is secured. The channeling scoops are claimed to preferentially direct water over the blade and tail fin, thus enhancing lift and thrust.

This is a modification of U.S. Pat. No. 6,375,531. The area of the foot-pocket is extended on both sides of each foot by means of “scoops” intended to increase the amount of thrust-producing water displaced during flapping. Actually, the increase of the area of the foot pocket (transverse to the direction of the flapping movement) will increase the power consumed by the swimmer, more than the amount of thrust produced. If an improvement is seen in the swimming speed, it is gained on the expense of disproportionately large increase of power expenditure.

A similar approach is taken in U.S. Pat. No. 7,614,928 (2009) to Grivna, which installs to both sides of each foot pocket two “lateral fins” for increasing the volume of the water moved by the flapping feet. The extra amount of water moved by the lateral fins will be ejected in directions which are not favorable for producing useful thrust, but a substantial drag increase will result.

U.S. Pat. No. 7,040,942 (2006) to Houck, describes a fin consisting of a sleeve which surrounds the lower leg (or the arm) of the swimmer, to which two fins are attached. The fins are extended laterally and form surfaces parallel to the legs and normal to the plane in which the flapping is performed. The fins as described will displace more water than the flapping legs alone. Being tied to the shins of the swimmer, only a small fraction of the water displaced will be pushed in the direction parallel to the axis of the body, and generate thrust. Their efficiency is diminished by the feet which “stick out” from between the fins and will disturb the direction of propelling streams.

In U.S. Pat. No. 7,083,485 (2006) to Melius, the swim fin is a blade shaped as a flat projection of a fish, (including pectoral fins, caudal fins and tail), on top of which the foot pocket is attached. The distal portion of the swim fin has the shape of a fish tail, claimed to produce lift/thrust, which propels the swimmer forward.

This is a further expansion of the U.S. Pat. Nos. 6,375,531 and 6,893,307. The soles of both feet of the swimmer's are attached to the flat blade. The sheens stick out of the plane of the blade at a large angle. The drag produced by the planar blade and the poor hydrodynamic profile of the foot pocket represent an important power waste. Also, a large fraction of the produced thrust will not be parallel to the axis of the swimmer's body and thus will not contribute to the movement in the desired direction. Several US patents, such as U.S. Pat. No. 6,146,224 (2000) to McCarthy, U.S. Pat. No. 6,379,203 (2002) to Kuo. U.S. Pat. No. 7,115,011 (2006) to Chen, U.S. Pat. No. 7,753,749 (2010) to Mun, describe fin blades which are provided with a variety of openings, through which the water can flow from one side to the other side of the blade. While the formation of such water streams is claimed to reduce the drag, it for sure reduces the efficiency of the conversion into thrust of the power used by the swimmer. Indeed, a portion of the water which was moved by the blade moves from the high-pressure side of the blade to the low-pressure side without producing thrust.

One of the advanced designs in the class of monofins is the PowerSwim device, developed by DEKA [Popular Mechanics, November 2007, p. 22], U.S. Pat. No. 7,988,508 (2011) to Langenfeld et al., which is based on the early concept of U.S. Pat. No. 3,122,759 (Gongwer). This device consists of two hydrofoils. One of them has a high aspect ratio (approx. 10), with a span of 1.80 m, which oscillates at about the midriff, in the front of the swimmer. The blade is connected to a lever which is fastened between the ankles and the knees of the swimmer. The second hydrofoil, of a smaller aspect ratio and span is fastened to, and oscillates right above the ankles of the swimmer. It is claimed that a swimmer using the PowerSwim device can achieve 150% higher velocity than with regular fins.

The device of U.S. Pat. No. 6,881,113 (2005) to Smith uses as blade a rigid hydrofoil, connected to the feet of the swimmer in a flexible manner. While the movement of the blade relative to the foot pockets is controlled, no effort is made to control the direction of the water streams produced by the flapping movement, around the “support structure” the feet or the rigid blade in order to increase the thrust or reduce the drag.

The main deficiencies of the fins of the prior art may be summarized as follows.

-   -   In order to increase the amount of water pushed by the fin, the         surface of the fin including the foot pocket is expanded either         laterally or axially. In both cases the gain in thrust is         accompanied by an even greater increase of the drag and of the         power which is consumed for flapping the fins.     -   In order to reduce the drag, a variety of gaps, holes, windows         are practice in the blade. Some of the water pushed by the blade         will escape through these holes thereby reducing the drag. In         the same time the power efficiency of the flapping is diminished         since less water will form the propelling jets which push the         swimmer forward. These opposed approaches indicate a lack of         understanding of the flow pattern needed for producing thrust by         the flapping feet and fins and of the methods to be used for         reducing drag.     -   The blade of the fin is usually parallel to the sole of the         foot. This results in difficulties in pushing the water         displaced by the fins in direction along the general axis of the         swimmer's body, and thus in reduced power efficiency. Several of         the fins of the prior art are designed so as to be easily worn         when walking. The angle between the blade and the foot sole         causes the water to be pushed by the blade in directions not         suitable for producing thrust.     -   Much of the water moved by the fins and monofins of prior art         forms streams which are not directed backwards and do not         produce thrust. Also, the shape and orientation of the foot         pockets leads to significant drag which consumes much of the         power spent by the swimmer.

Power Efficiency.

In order to generate propulsion which results in velocity, human swimmers as well as fishes or aquatic mammals need to spend power (energy per unit time). For the present discussion only the power consumed by the swimmer for moving the legs will be considered.

Cetaceans and fishes of the tuna or marlin families produce propulsion by the oscillating movement of the rear ⅓rd part of the body (carangiform and thunniform movement), while the front part of the body does not oscillate practically at all. Human swimmers can use the same swimming style. However, the movement of the body of human swimmers using swim fins and practicing the “dolphin kick” style is closer to the “anguiliform”, where the propulsion is produced by a wave-like movement of the body. The body bends in a progressive manner as if a wave passes through it, from the head to the end of the toes (and the end of the swim fin). In the animal realm, the anguiliform style is less efficient and results in lower velocities than the carangiform or thunniform movement.

The mechanism by which flapping tails or fins generate propulsion is as follows: at each up- or down-stroke, or half cycle of the flapping of the tail or fin (by a cetacean or by a human swimming face down or on the back), a certain amount of water is pushed backwards and as reaction to it, the body of the swimmer is propelled forwards. The water is pushed as a backwards-directed stream (jet). The force (thrust) by which the swimmer is propelled forward at each stroke of the fin depends on the size, shape and orientation of the blade during the stroke, the amplitude and frequency of the flapping movement and on the flow conditions (water streams) produced as result of the flapping movement in the space close to the fin and to the body of the swimmer.

Despite the ability to closely copy the shape, size, elasticity, texture and movement of the flukes of cetaceans [References: Bose, N. et al. Proc. R. Soc. London B 242 (No. 1305) pp 163-173(1999), Fish, F. E. et al., Bioinsp. Biomim. 1 (2006) pp 42-48] and of tails of fast swimming fishes, the swimming performance of the man-made swim fins is not matching that of the model animals. The efficiency of converting the power consumed by the swimmer into propulsion is much lower for humans than for the aquatic animals used as models for designing the swim fins. For the same power consumed, a human swimmer will generate less propulsion than a dolphin of the same weight and height.

The power consumed or used by the animal or human swimmer can be perceived as consisting of several components:

Pt (total power consumed by swimmer)=Pp (power used for generating thrust)+Pd (power used for overcoming the body drag)+Pw (power wasted during the flapping of the legs and swim fin).

For a given swimmer and conditions, Pt is given. In order to maximize Pp, one needs to minimize Pd and Pw. Only small reduction of Pd has been achieved by using body suits, such as Spandex. The swim devices covered here address the reduction of Pw.

The power wasted Pw, can be seen as having two components:

Pw=Ppf (power wasted by producing parasitic, i.e. non-thrust producing water streams)+Pdf (power wasted to overcome the drag of the flapping feet)

The designs of the swimming devices of prior art have no methods for reducing the parasitic (called here also “shunt” streams or currents) streams, which do not produce thrust, or for reducing the power wasted as drag, during the flapping of the legs.

DESCRIPTION OF THE FIGURES

FIG. 1. Shows in a simplified manner the main water flows produced by a moving plate and the parasitic (shunt) currents for conditions not according to this patent.

FIG. 2 a, 2 b, 2 c, show the main outlines and movements of a blade connected to the end of a flapping rod (2 a), cylinder (2 b) and cone (2 c).

FIG. 3 a, 3 b. Show the water streams set in motion by a swim fin of this invention, shown as a ghost side view in two positions of a flapping foot and a ghost plan view of a foot wearing a swim fin.

FIG. 4 is a ghost plan view of the monofin of this invention, with feet held parallel in individual foot pockets, the narrow (necking) distal end of which is fastened to a common blade (monofin).

FIG. 5 shows a ghost plan and a side view of a monofin of this invention, (crossed feet), for achieving a reduced frontal area of the cowling, and a lower drag.

FIG. 6 shows a ghost plan and a side view of a monofin of this invention, (parallel feet). Letters M, N, P and R refer to transverse cross-sections through the cowling, at the levels indicated.

THE POWER WASTED DURING THE FLAPPING OF SWIM DEVICES

There are two main ways in which a portion of the power expended by the swimmer for moving the flapping legs/fins is wasted (Pw).

A. Power Wasted as Parasitic Streams (Ppf).

An important fraction of the expended power is wasted by the blade setting in motion a mass of water which will not be part of the thrust-producing jet (Ppf), as discussed below.

Here, the convention is made to call “in front” or “ahead” a position facing, or streams flowing in the same sense as the instantaneous movement of the fin, or adjacent to the “front” side of the fin. Positions or streams flowing in the opposite sense of the instantaneous movement of the fin are called “behind”, as they are adjacent to the “back” side of the fin. The front and back of the fin switch at every up- and down-stroke of the fin.

For a swimmer in “ventral” position, the flapping action is divided into a “dorsal” or “up-stroke” and a “ventral” or “down-stroke”.

During both the up- and down-movements of the swim fin, a zone of transient, high dynamic pressure (caused by the movement of the fin) forms in front of the fin. Simultaneously, a zone of lowered dynamic pressure is produced behind the fin. The push exerted by the fin produces powerful stream of water (propulsion-generating or propelling jet) directed backwards, away from the body, along a path which is generally parallel to the axis of the swimmer's body. The size and direction of these water jets is controlled by the pattern in which the blade moves (flapping amplitude, orientation) and by the size and design (shape, profile) of the blade. Propulsion of the swimmer's body results as reaction to these propelling jets of water.

Not all the water corresponding to the volume swept by the fin during a half stroke will be part of the propelling jets. Observations show that portions of the water mass pushed by the dorsal or ventral sides of the swim fin (during the dorsal or ventral movement, respectively) separate from the direction of the main streams and turn around the borders of the swim fin to flow “backwards”, into the space which during that half of the flapping cycle is “behind” (i.e. ventral or dorsal respectively) the moving fin. These streams do not produce propulsion. Thus, the power which was spent for accelerating the corresponding water mass is wasted.

The shunt streams flow from the high pressure zone produced ahead of the plate, during its movement, around the borders of the plate, into the low-pressure zone generated behind the plate. FIG. 1 depicts the zones of transient dynamic high pressure and the flows generated by a flapping plate, simulating a fin not of this patent, submerged in water. A flapping plate 1 with a central axis 4 is connected to a lever 2, having an axis line 5, by means of a hinge 3 which allows for a limited rotation of plate 1, to maximum angles S and S′ respectively, on both sides of the position in which the axis 4 of plate 1 is parallel to the axis 5 of lever 2. The flapping movement is caused by lever 2 pivoting around a pivot point P to positions corresponding to angles V and V′, on opposing sides of a neutral position indicated by a line N. For reasons of simplicity, only the “down” stroke of the flapping cycle is shown, as indicated by an arrow 6. The “up” stroke (shown in dotted lines) is symmetrical with respect to the neutral axis N.

In the drawing, lever 2 is at the maximum amplitude (measured by angle V), during the oscillating (flapping) movement about the neutral position N. As it moves downwards, plate 1 exerts in front of it (in the direction of the movement), a push on the water, manifested as a dynamic pressure increase in the zone marked “in front”, which forms a jet, moving as indicated by the arrows 8. As result of the movement of plate 1, a zone of lowered pressure is formed on the opposite side of the blade, in the zone marked “behind”. In order to balance the pressure difference between the two sides of plate 1, a portion of jet 8 will leave the high-pressure zone, will turn around the borders of plate 1 and will enter the low-pressure zone, as parasitic, or shunt streams 9. The water contained in shunt streams 9 is subtracted from the water which was initially part of jet 8 and thus, reduces the size of the propelling jet. As result, the power consumed by the swimmer for accelerating the water ending as shunt streams 9 is wasted.

In none of the swim devices of the prior art are there attempts made to reduce the amount of wasted power, by reducing the streams which flow from the front to the back of the flapping blade and diminish (shunt) the flow of water which generates propulsion.

B. Power Wasted as Drag (Pdf).

A significant portion of the power consumed by the swimmer is lost for overcoming the drag associated to the flapping of the legs. In order to advance, a human swimmer will flap the feet/legs in the sagittal plane of the body, dorsally and ventrally (if the body is in prone position, this will correspond to up and down). Hydrodynamic studies have shown that cylinders (and by analogy, the sheens and legs of the swimmer) moving in water produce significant drag if the direction of the movement is normal to the axes of the cylinders (cross-flow). (Reference: S. F. Hoerner, “Fluid Dynamic Drag” 1965. Library of Congress No. 64, 1966). The drag is proportional to the “frontal area” and to the square of the velocity of the water in cross-flow over the cylinders. The frontal area is defined as the largest cross-section of the submerged body, normal to the direction of the water flow. The drag is also strongly dependent on the shape of the submerged body. A body with a hydrodynamic cross section e.g. elliptical, with the long axis parallel to the direction of flow, and the short axis perpendicular to it, with the ratio of the long axis A to the short axis B of A/B=2 will experience only approx. 70% and for A/B=4, only 50% of the drag of a circular cylinder with the diameter equal to the axis B of the ellipsoid.

None of the swim fin designs of prior art, addresses in specific manner the reduction of the drag generated by the flapping movement of the legs of the swimmer, by giving a hydrodynamic shape to the foot pocket. On the contrary, many of the fins of the prior art increase the frontal area of the foot pocket in the attempt to increase the volume of the water pumped at each stroke. This results in a moderate increase of the thrust but in a much higher increase of the drag.

EXPERIMENTS

The existence of an interaction between the blade and the flapping feet/legs of the swimmer was investigated by means of simplified models. A blade mimicking a dolphin fluke was built at a reduced scale of 1/10 and was attached by means of a hinge to an elongated body, simulating the feet/legs of a swimmer which was oscillated to an angle “V” of about 35 degrees up and down, around a pivoting axis “PA”, perpendicular to the longitudinal axis “LA” of the elongated body. (FIG. 2 a,b,c). The hinge allowed the blade to deviate to an angle “S” of about 35 degrees, on both sides of the neutral position defined by the axis of the elongated body. Three shapes were tested for the elongated body: a thin rod (FIG. 2 a), a circular cylinder (FIG. 2.b) and a cone with an apex angle “A” of 35 degrees (FIG. 2.c). The diameter of the cylinder and the maximum diameter of the cone were approx. 30% of the wingspan of the blade. The models were submerged in a container filled with static water. Each model was separately oscillated manually (so as to simulate the flapping of the feet/legs of a swimmer) and the movement of water in the neighborhood of the elongated body and the blade was observed by following the movements of small neutrally buoyant particles suspended in water.

The oscillation of the blade attached to the rod of FIG. 2 a produces water streams (jets) directed away from the blade, more or less along the median position of the axis of the oscillating rod. No significant movement was visible along the rod.

For the case of the blade connected to the cylindrical body (FIG. 2 b), significant turbulence, with erratic currents were seen along the oscillating cylindrical body, which interacted with the water displaced by the blade, generating a less structured movement pattern of the water (weaker jets along the axis of the body).

The blade connected to the apex of the oscillating conical body (FIG. 2 c) produced the strongest axial jets and significantly less turbulence around the oscillating body, than when connected to the cylindrical body. A careful observation indicated that at each half-stroke, water close to the oscillating conical body moved from the side “ahead” of the cone (high dynamic pressure), towards the apex and across the axis of the cone, in the narrowed portion, and to the side “behind” the blade (low dynamic pressure). The direction of these “cross-over” streams changed at each half-stroke, but the water jets leaving the blade along the median position of the axis of the body were stronger than for either of the other models. This is an un-expected finding, not mentioned before in the rich scientific literature on propulsion by oscillating foils of marine animals, humans or crafts. It led to two conclusions:

-   -   (1) in order to improve the swimming efficiency one needs to         control not only the shape and movement of the swim device         itself, but also the flow pattern, that is, the water streams         around the flapping feet in the zone upstream of the blade. The         flow pattern is improved by shaping the part of the swim device         upstream of the blade, to make possible the rapid filling of the         low-pressure zone formed behind the blade at each half-stroke.     -   (2) The foot pocket should be shaped so as to produce the least         possible drag and not to “pump” water, as in most of the prior         art devices.

A mechanism by which the cross-over currents lead to increase efficiency of the conversion of the energy expended for flapping into thrust will be discussed below. (FIG. 3 a, b). As it will be shown, the present invention describes swim devices of a design, which has no antecedent in the prior art. The design is based on new insight gained from own above experiments on the water streams generated by the flapping movement.

OBJECT OF THE INVENTION

Consequently, this invention addresses swim devices which are shaped with the objectives of improving the overall efficiency of converting the power exerted by a swimmer into thrust, by favoring the formation of producing of currents which will cross over the long axis of the flapping feet and balance the dynamic pressure difference between the two sides of the flapping blade, thereby resulting in less wasted energy by the shunt currents, as well as in less the drag experienced by the flapping legs and feet.

DETAILED DESCRIPTION Preferred Embodiments

The swim devices of this invention achieve these objectives and comprise:

-   -   A structure of a generally prolate shape (called foot pocket)         which surrounds and is fastened to each foot of a human swimmer.         The proximal end of each of the foot pockets is provided with an         opening through which the foot enters. The distal portion of the         foot pocket is shaped as a taper, with the transversal         cross-section of preferably elliptical shape, having the long         axis parallel to the plane of the flapping movement. The taper         ends distally in a “necking” to which the blade is attached. The         necking has a substantially smaller frontal cross-section than         the rest of the cowling. The cowling has transversal         cross-sections of hydrodynamic shapes and possesses sufficient         rigidity, to maintain its shape during the oscillating movement         imposed by the flapping feet.     -   Each foot pocket may be attached to one individual blade. The         two swim fins so obtained allow the swimmer to move the feet         separately, in flapping movement.     -   The foot pockets for the two feet may be attached to one common         blade, building a monofin.     -   A structure of a generally prolate shape hereinafter called         “cowling”, which surrounds both feet of a human swimmer. The         proximal end of the cowling is provided with an opening through         which the feet and possibly also a portion of the legs of the         swimmer are accommodated. The cowling is attached distally to a         blade, thus forming a monofin.     -   A taper, at the end section of the cowling which extends         distally along a certain axial distance along the feet and         beyond the soles of the swimmer, to a “necking” with a         substantially smaller frontal cross-section than the rest of the         cowling. The cowling has transversal cross-sections of     -   hydrodynamic shapes and possesses sufficient rigidity, to         maintain its shape during the oscillating movement imposed by         the flapping feet.     -   A blade of predetermined shape, size and profile, connected to         the necking of the tapered end of the cowling, and capable to         oscillate in the sagittal plane, to predetermined angles, on         both sides of a neutral position, in a manner which generates         thrust. The blade may be of any desired shape, preferably         mimicking the fluke of dolphins, the lunate tail of tuna,         marlin, etc., as well as a hydrofoil with straight or curved         leading edge.     -   A connecting means (such as a lever or similar device) for         assembling in a functional relationship the blade to the necking         in the taper of the foot pocket or to the cowling and to the         feet and legs enclosed therein, of the swimmer.

The drawings of FIGS. 3 a and 3 b show fins of this invention worn on each foot. The drawings of FIGS. 4, 5 and 6, depict the monofins of this invention.

In FIG. 3, a foot pocket 11, surrounds each foot 10. Distally, foot pocket 11 forms a taper 17, ending in a necking 13, with a hinge device 16, to which a blade 12 is attached. Inside foot pocket 11, foot 10 is fastened to a foot plate 14 which is connected to a lever 15, attached by means of hinge device 16 at necking 13 to blade 12. The flapping takes place in the sagittal plane of the swimmer, to angles V and V′, of approx. 30-50 degrees on each side of a neutral position defined by the longitudinal axis of the swimmer's body. The up-stroke and down-stroke movements of the fins generate a thrust 19 and cross-over streams 18, as indicated. The transversal cross-section of foot pocket 11 is of hydrodynamic shape, as indicated by the letters M, N, P and R, at the marked position along the length of the foot pocket. The angle T of taper 17 is of 35-50 degrees, as shown in FIG. 2 b.

A further embodiment of this invention is shown in FIG. 4. Each foot 20 is surrounded by a foot pocket 21, which is identical to that described in FIG. 3. Each foot pocket 21 extends beyond the toes of the swimmer as a taper 23, ending as a narrow necking 24, provided with a hinge device 27, to which a single blade 22 is attached, thus resulting in a monofin. Inside each of foot pockets 21, a foot plate 25 is located, to which feet 20 are fastened in a functional manner. From each foot plate 25, a lever 26 is extended distally, through neckings 24, and connected to blade 22 by hinge device 27. In plane view, the angle T of tapers 23 is of 35-50 degrees.

Another embodiment of this invention is a monofin where both feet are fastened within a single containment, called cowling, depicted in FIG. 5. A cowling 31 surrounds the feet 30, possibly extending to the level of the ankles of the swimmer. In order to reduce the volume and the frontal area of the cowling, the feet are placed in overlapped position (crossed). Cowling 31 extends beyond the toes of the swimmer as a taper 32 which ends as a narrow necking 34, to which a single blade 33 is attached. A foot plate 35 located within the cowling is connected to a lever 36, which passes through necking 34 and attaches to blade 33. In plane view, taper 32 encloses an angle T of preferred 35-50 degrees.

In a variation of this embodiment, the cowling can extend from the region of the ankles up, to the region of the knees, as marked by 37. The transversal cross-section of extended cowling 37 has a hydrodynamic, low drag shape.

A still further embodiment of this invention is a monofin where both feet are fastened within the cowling, depicted in FIG. 6. Thus, cowling 31 surrounds feet 30, possibly extending to the ankles of the swimmer. In this configuration, the feet are placed in parallel position (side by side). Cowling 31 extends distally beyond the soles of the swimmer as taper 32 which ends as narrow necking 34, to which single blade 33 is attached. Foot plate 35 located within the cowling is connected to lever 36, which passes through necking 34 and attaches to blade 33. In plane view, taper 32 encloses an angle T of preferred 35-50 degrees.

In FIG. 5, transversal cross-sections M, N, P and R depict the shape of extended cowling 37 at the indicated positions. When progressing distally, from the rim of the cowling (in the region of the knees), the shape of the cowling changes from approximately circular to elliptical. The approximate ratio A/B of the long axis A (in the sagittal plane), to the short axis B (in the frontal plane) of the ellipses vary from A/B=1 (corresponding to a circle) at position M, to A/B=1.3 at position N, A/B=1.5 at position P and A/B=4.0 at position R.

The Foot Pocket and the Cowling.

The foot pockets cover individually each foot of the swimmer up to the region of the ankles and confer to them a hydrodynamic shape which generates low drag, during the flapping movement. The foot pockets may be attached each to one blade, as in regular swim fins, or both to one single blade, resulting in a monofin.

The cowling covers the joint feet and a predetermined length of the flapping legs. The cross-section of the cowling is designed with a hydrodynamic shape, for minimizing the drag encountered during the flapping movement.

More or less of the length of the legs may be covered by the cowling, as illustrated in FIGS. 5 and 6. In one embodiment, the cowling covers only the feet, and its proximal rim is located in the region of the ankles. The cowling may extend proximally to about the region of the knees, as marked by 37, in dotted lines. The proximal rim of the cowling should not impede the flexing of the knees, required for the flapping movement. The more of the length of the legs is covered by the cowling, the smaller will be the overall drag encountered during flapping.

In a further embodiment, in order to reduce the frontal area, of the flapping cowling, the swimmer may assume a posture with feet crossed. (FIG. 5). In this posture, the coronal cross-section (frontal area) is smaller than for the posture with parallel feet (FIG. 6). The drag resulting by flapping a cowling sized for accommodating the crossed feet of the swimmer, will be less than by flapping a cowling designed for parallel feet.

When assuming the crossed feet position, either foot can be placed above (ventrally) the other one, which is placed below (dorsally).

Inside the foot pockets 11 (FIG. 3 a) and 25 (FIG. 4) and inside cowling 31 (FIGS. 5 and 6), means are provided for attaching the legs to the blades (respectively 12, 21 and 31) in a functional relationship. One or both feet may be attached to the blade, by means of a suitable device, such as a foot plate (respectively 14, 25 and 35), and a connecting lever 15, 26 and 36, respectively. In a preferred arrangement, only one of the feet, namely the one which is located dorsally is fastened to the foot plate 35 in FIG. 4. In order to assure a proper division of the power load between the two legs, in a preferred arrangement, the feet or the legs are connected to each-other by any convenient means (not shown), including strips of material (moldable after the shape of the feet, stretchable or not, etc.) making possible the secure fastening of the foot pocket or cowling to the legs, without producing unduly inconvenience to the swimmer.

The cross-sections M, N, P and R of FIGS. 3 b and 6 show the shape of the walls in the transverse plane at three positions along the foot pocket and respectively the cowling, for the posture with crossed feet. At all levels, the frontal area (projection in the coronal plane) is smaller for the crossed-feet posture than when the feet are held parallel to each-other.

The Taper.

As depicted in FIGS. 3 b, 4, 5 and 6, the distal section of the foot pocket and of the cowling (respectively 11, 21, 31, 31), beyond of the toes of the swimmer, assumes a taper (respectively 17, 23, 32, 32) which is one of the important embodiments of these swim devices. The taper decreases the frontal area of said foot pocket and said cowling from that needed to contain the swimmer's foot (as specifically indicated as sections N and P, in FIGS. 3 b and 6), to a small fraction thereof, corresponding to neckings (respectively 13, 24, 34, 34, in FIGS. 3 a/3 b, 4, 5 and 6), and also shown in cross-section, as section R in FIGS. 3 b and 6.

With reference to FIGS. 3 b and 6, the maximum width of the foot pocket, and of the cowling, in frontal cross-section as shown at position R should represent 5-15%, preferably not more than 10% of the width of the cross-section at position N. While a very narrow necking is desirable, the width at R should be sufficient for containing lever (positions 15, 26, 36, 36) or any other means for attaching in a functional manner the blade to the foot pocket of the swim fin or to the cowling of the monofin and to the feet of the swimmer.

As seen in the frontal sections in FIGS. 3 b, 4, 5 and 6, the angle T, enclosed by taper (respectively 17, 23, 32, 32), is less than 90 degrees, preferably 30-50 degrees, most preferably 35-45 degrees. In the sagittal plane, the value of the angle enclosed by the taper is less critical. It is determined by the dimensions of the lever or other means used for connecting the foot pocket and cowling to the blade. The shape of the transversal cross-section of cowling 11, 21, 31, 31 is conveniently defined by the ratio of the major axis to the minor axis, A/B of the ellipse. As exemplified in FIG. 6, but valid for all drawings of this invention, this ratio changes from approximately A/B=1.5, as shown in section P, to that of section R, where the ratio is approximately B/A=4.0. This last value is known to generate least drag, when placed in a flow stream, parallel to axis A. (Hoerner, loc. cit.).

The Blade.

Blades (12, 22, 32, 32 of FIGS. 3 a/3 b, 4, 5 and 6, respectively), of several shapes may be used as part of the swim devices described here. Preferred shapes include blades which mimic the flukes of cetaceans (FIGS. 3 a/3 b, 5,6), tails of fast-swimming fishes, such as tuna (FIG. 4), marlin, sailfish, as well as hydrofoils with straight or swept leading edges, or any other shapes, as known in the art. Preferably, these blades are provided with hydrodynamic chord profiles, in order to generate low drag and good lift, as known in the art.

The aspect ratio AR=(L̂2)/S (fin span L, squared divided by the plane area S, of the fin) of the preferred fin is larger than 1. For fins using hydrofoils, the aspect ratio is preferably AR=2-6. The blade may be rigid or may posses a certain span-wise and chord-wise flexibility. The materials of which blades may be made include synthetic rubber silicon rubber (both of which may be reinforced by adequate spars or spines), polyurethanes, polyesters, etc. By incorporation in the structure of the blade of reinforcing ribs it is possible to control the elasticity, resilience and degree of deformation during flapping, especially for larger AR values. The materials of the ribs include metals, especially wires or bars of metal, plastics, battens made of glass-reinforced plastics, or of carbon fibers-reinforced plastics, which will confer to the blade the desired elasticity and sturdiness etc.

Swim fins or monofins of this invention provide excellent propulsion for blade spans of 0.3-0.5 m, which is in the same range as the fluke spans of cetaceans with body weights and body-lengths similar to those of human swimmers. [References: Bose, N. et al. Proc. R. Soc. London B 242 (No. 1305) pp 163-173(1999), Fish, F. E. et al., Bioinsp. Biomim. 1 (2006) pp 42-48],

Connecting the Parts.

With reference to FIGS. 3 a/3 b, 4, 5 and 6, the blade is attached to the feet of the swimmer and to the foot pocket or the cowling by means of a footplate 15, 25, 35, 35, respectively to which the feet 10, 20, 30 and 30, respectively are fastened, and one end of a lever 15, 26, 36, 36, respectively which, in its simplest form is a bar. The other end of the lever is preferably attached to the area of the center of the leading edge of the blade. Possibility may be incorporated to adjust the angle between lever and foot plate, to accommodate individual conformations of the feet of the swimmer. The lever may posses a certain degree of elasticity for bending, but should be quite un-yielding to torsion or rotation around its axis.

The blade may be attached to the lever by a hinge means which may be a rigid or elastic connection, as known in the art. Spiral springs, torsion bars or slit pipe springs may be used to this effect, as well as rubber or plastic parts which possess the desired properties. The blade follows the flapping movement of the foot pocket or cowling and, as reaction to the dynamic pressure it exerts on the water, bends or pivots around said hinge means or in its absence, around a pivot point in the necking, to limiting angles schematically depicted in FIG. 1 as S and S′, about its neutral position N. The flapping movement of the foot pocket and of the cowling and the bending of the blade, take place in the dorso-ventral (sagittal) plane. The degree of bending allowed by the blade-lever connection depends on the type of blade used. For blades mimicking the flukes of cetaceans, the plane of the blade may swing both sides of the neutral position to a maximum angle of 45-50 degrees, although angles of 25-45 degrees seem to confer efficient propulsion.

For blades mimicking the tail of tuna (FIG. 3 a/3 b, 4) or for hydrofoils of large aspect ratio, the connection between the blade and the lever may be suitably rigid. The dynamic pressure generated by the flapping movement, will cause a certain degree of span-wise and chord-wise bending and twisting of the blade itself, which will lead to efficient propelling action.

Other methods for connecting the blade to the feet of the swimmer, can be practiced so as to fit specific material or design peculiarities. For example, by building the foot pocket and the cowling including the taper suitably sturdy, it is possible to connect the blade directly to the necking at the distal end of the taper of the foot pocket or cowling, to which the swimmer's feet are attached. This is a suitable procedure for achieving a functional connection between the feet of the swimmer and the blade of the swim devices covered here.

The Fastening Devices.

The swim fin or the monofin are preferably built in a manner which allows them to be rapidly attached to the feet/legs of the swimmer, and rapidly disconnected from them. According to a preferred construction, the foot pocket or the cowling consist of two or more parts, which can be attached to each-other along corresponding matching sides. In a more preferred construction, the foot pocket and the cowling are made of two parts which have matching sides along the legs of the swimmer, in the coronal plane. The foot pocket or the cowling are thus divided into a dorsal and a ventral part. Both parts may accommodate devices to attach the parts to each other, as well as to the feet or legs of the swimmer, and to the footplate, lever and blade itself. Said devices, will allow for a rapid assembly and a rapid attachment and fastening of the foot pocket and cowling, inclusive blade to the legs of the swimmer and a rapid detaching and removal thereof. Preferred devices may be fasteners, flexible and/or elastic tapes which may be provided with the possibility to adjust the fastening tension, such as clamps, Velcro closures, buckles fasteners, etc., as known in the art. For the case of monofins, it is preferred that the feet be attached to the foot plate in the cowling, one at the time. Inside the cowling, at several convenient levels, supports may be provided for means of further fastening if so needed, the legs of the swimmer to each-other, to the cowling, and to the foot plate.

In a preferred embodiment, within the taper, supports (not shown) are provided for attaching and guiding the foot plate and the lever in the desired orientation for suiting the individual preference of the swimmer.

Materials and Construction.

The details of the design of the foot pocket and of cowling and the materials of construction can vary. The thickness of the shell is function of the material used in its manufacture. The foot pocket and the cowling have to be sufficiently sturdy to maintain their shape during the flapping movement. Adequate materials for making the shell are: wood, metals (aluminum etc.), plastic materials (such as PVC, polyethylene, polypropylene, ABS, polyamides, polyurethanes, polyesters, etc.) glass-reinforced or carbon fibers-reinforced plastics, etc.

The proximal rims of the foot pockets or cowling may be protected by a soft and cushioning material which will prevent injuring the legs/feet of the swimmer and will ensure a smooth (low drag) transition for the water flowing along the legs, to the outer surface of the cowling.

The space not occupied by the feet and the legs within the foot pocket or cowling is preferably filled with materials to ensure a tight connection between the feet and the cowling, without however, unduly cramping the feet and legs. Preferred materials are molded foams including rubber, polyethylene, poly-propylene, poly-styrene, poly urethanes, etc. bags or pouches which may be inflated with gas or filled with liquid, to achieve a suitable tight fit (no play) between the foot pocket or the cowling and the foot/feet/legs of the swimmer. The overall specific gravity of the foot pocket and of the cowling as installed, including the legs, may be lower, equal or higher than of water. A specific gravity close to that of water is preferred, in order to confer neutral buoyancy to the body of the swimmer. Added materials (preferably inside the foot pocket or the cowling) can be used in order to obtain the desired buoyancy.

In a further embodiment, the foot pocket and the cowling can be built as massive entities (block, monolith) of the desired outside shape, without a distinct shell, and provided with internal cavities for accommodating the feet (and legs) of the swimmer, as well as the means for connecting the legs to the blade. The block may be made up of several shaped parts which are assembled together and allow for the rapid fastening to the legs of the swimmer, and removal there-from of the swim fins or monofin. This type of foot pocket and of cowling may be made of a convenient plastic material provided with voids (foam) in order to have the desired overall specific gravity and be sufficiently sturdy, as required for maintaining the prescribed shape when undergoing the flapping movement.

Making the Fin and the Monofin.

The fabrication of the parts which make the fin and the monofin is not a problem for someone skilled in the operations of molding and casting plastic materials. The foot pocket and cowling may be fabricated by operations including vacuum forming, hot pressing or similar techniques. The details of the operation depend on the properties of the material selected. The taper is best fabricated as integral part of the foot pocket and of the cowling, although other options are possible. The blade fabrication includes methods such as casting (of silicon rubber or similar), injection molding, forming by sculpturing or by bonding together pre-formed sub-components. The other components, such as fasteners, tying tapes, foot plate, lever, etc. can be either purchased on the market or fabricated by using broadly available materials and skills.

Preferred Embodiments Function

The taper at the distal end of the foot pocket and of the cowling is of essence for improving the power efficiency of propulsion generation by the flapping swim fin and monofin described here. The simplified typical water flow patterns produced by the fin and monofin of this invention are sketched in FIGS. 3 a/3 b and 5, and marked as “cross-over streams.

With reference to FIG. 3 a, the movement of the swim fin, as shown by arrows marked “UP-STROKE” and “DOWN-STROKE” causes a zone of high dynamic pressure to form in front of blade 12, as origin for propelling jet indicated by the arrows marked “THRUST”. A zone of lowered dynamic pressure will form behind blade 12. The forward movement of the swimmer combines with the flapping movement of the foot pocket or cowling to generate streams of water 18, which move distally, and as allowed by the presence of taper 17, cross the frontal plane of the foot pocket or cowling in the zone of necking 13, and flow into the zone of dynamic low-pressure formed behind blade 12, thus equalizing the pressure difference between the two sides of the blade. In this manner, the pressure driving force which caused the formation of the non-propelling parasitic or shunt streams (marked as 9, in FIG. 1) is removed. Since less water will form the parasitic streams, more of the water pumped by blade 12 will remain as part of the thrust-producing propelling jet.

While the direction of the flows across the taper and necking alternates for each half-stroke, the resulting propelling jets have the same general direction. The stronger jets will produce more thrust and higher power efficiency for the swimmer than other devices where balancing the dynamic pressure difference around the two sides of the blade is performed by shunting streams flowing around the borders of the blade or through openings practiced in the blade itself.

In conclusion, the swim devices covered here are built with a hydrodynamic and tapered shape which, at each half-stroke of the flapping feet, will cause that water from the higher dynamic pressure area, in front and up-stream of the foot pocket or cowling, moves across the necking of the taper, into the area of lowered dynamic pressure, behind the flapping blade, thereby improving the efficiency of converting the power expended by a swimmer into thrust, and reducing the drag encountered during flapping, compared to the devices of prior art. The swim device, whether fin or monofin of this invention consist of the following parts:

-   -   A foot pocket of elongated shape, which surrounds each foot of         the swimmer, or a cowling of elongated shape which surrounds         both feet and a certain length of the legs of the human swimmer.     -   A taper of the distal end section of the foot pocket or of the         cowling ends in a necking which has a substantially smaller         width than the maximum width of the foot pocket or of the         cowling, respectively. At the necking, the foot pocket or the         cowling is connected to the blade, preferably to the leading         edge thereof. The foot pocket and the cowling have transversal         cross-sections of hydrodynamic shape which results in a low         drag.     -   A blade of predetermined shape, size and profile, connected in a         functional relationship to the necking of the taper of the         flapping foot pocket or cowling. The blade acts upon the water         for generating thrust which efficiently propels the swimmer         forward.     -   A lever or other means for connecting in a functional         relationship the blade to the feet of the swimmer and/or to the         foot pocket and to the cowling.     -   The swim devices possess sufficient rigidity, to maintain the         desired shape during the oscillating movement imposed by the         flapping feet.         Components of the Swim Devices, Fin and Monofin of this         Invention.

FIG. 1

-   -   1 moving plate     -   2 lever     -   3 hinge     -   4 axis of moving plate 1     -   5 axis of lever 2     -   6 direction of flapping movement     -   8 direction of thrust (propelling jet)     -   9 parasitic (shunt) streams     -   IN FRONT: zone of increased dynamic pressure     -   BEHIND: zone of decreased dynamic pressure     -   V, V′ flapping angles between lever 2 and neutral axis N, during         down-stroke and respectively, up-stroke     -   N neutral axis; longitudinal axis of body of swimmer     -   S, S′ angles between the axis of lever 2 and axis of moving         plate 1, during down-stroke, respectively up-stroke.         FIGS. 2 a, 2 b, 2 c.     -   A Apex angle of cone body     -   LA Longitudinal axis of flapping body     -   N Neutral position of flapping body     -   P Pivot point     -   PA Pivoting axis     -   S, S′ Angle between blade and LA during down-stroke,         respectively up-stroke.     -   V, V′ Angle between LA and neutral position N, during         down-stroke, respectively up-stroke.         FIGS. 3 a, 3 b     -   10 foot     -   11 foot pocket     -   12 blade     -   13 necking     -   14 foot plate     -   15 lever     -   16 hinge     -   17 taper     -   18 cross-over streams     -   19 direction of thrust     -   M,N,P,R transversal cross-sections at the indicated locations         along the foot pocket     -   T angle of taper     -   UP, DOWN: direction of flapping movements

FIG. 4

-   -   20 feet     -   21 foot pocket     -   22 blade     -   23 taper     -   24 necking     -   25 foot plate     -   26 lever     -   27 hinge     -   T taper angle

FIG. 5,6

-   -   30 feet     -   31 cowling     -   32 taper     -   33 blade     -   34 necking     -   35 foot plate     -   36 lever     -   37 extended cowling     -   38 hinge     -   39 cross-over streams     -   A long axis of ellipse     -   B short axis of ellipse     -   M,N,P,R transversal cross-sections through the cowling, at the         indicated locations     -   T taper angle 

What is claimed is:
 1. A swim device worn on the feet of a swimmer, for producing thrust by a flapping motion of the feet, comprising: (a) a foot pocket of elongated shape and hydrodynamic cross-section, surrounding each foot of the swimmer, (b) a taper of the distal end section of said foot pocket, extending distally beyond the sole of the feet of the swimmer and ending as a necking of substantially smaller width than the maximum width, of said foot pocket, normal to the plane of the flapping movement, (c) a blade of predetermined size, shape and profile, connected to said taper in a functional relationship, whereby during each half-stroke of the flapping feet, water from the higher dynamic pressure area, in front and upstream of the foot pocket is made to move across the necking of the taper, into the area of lower dynamic pressure, behind the flapping blade, thereby a more efficient conversion of the power expended by the swimmer into thrust is achieved and less power is wasted as drag, than in absence of these components.
 2. The swim device of claim 1, wherein said foot pocket has transversal cross-sections of hydrodynamic shape, such as an ellipse, with the long axis parallel to the plane of the flapping movement.
 3. The swim device of claim 1, wherein a frontal cross-section of said taper in said distal end section of said foot pocket encloses an angle of less than 90 degrees, preferably between 35 and 50 degrees.
 4. The swim device of claim 1, wherein said necking of said taper has a transversal cross-section the width of which is not more than 25%, preferably not more than 15% of the largest width of the foot pocket in a direction normal to the plane of the flapping movement.
 5. The swim device of claim 1, wherein each said foot pocket on the two feet of the swimmer is connected by the respective said necking to one blade, in a functional relationship.
 6. A swim device, comprising: (a) a cowling of generally prolate shape, surrounding both feet of the user, (b) a taper of the distal end of said cowling, extending distally beyond the soles of the swimmer and ending as a necking of substantially smaller width than the maximum width of the cowling, in a direction normal to the plane of the flapping movement, (c) a blade of predetermined size, shape and profile, connected to said taper in a functional relationship, whereby during each half-stroke of the flapping feet, water from the area of higher dynamic pressure, in front and upstream of the cowling is made to move across the necking of the taper, into the area of lower dynamic pressure, behind the flapping blade, thereby a more efficient conversion of the power expended by the swimmer into thrust is achieved and less power is wasted as drag, than in absence of these components.
 7. The swim device of claim 6, wherein said cowling extends proximally a certain length above the ankles of said swimmer, whereby a further reduction of the drag encountered by the flapping feet is achieved.
 8. The swim device of claim 6, wherein a frontal cross-section of said taper in said distal end section of said cowling encloses an angle of less than 90 degrees, most preferably between 35-50 degrees.
 9. The swim fin of claim 6, wherein said necking of said taper of said cowling has a transversal cross-section the width of which is not more than 25%, preferably not more than 15% of the largest width of said cowling, in a direction normal to the plane of the flapping movement. 